Abstract:
A full scale, three dimensional (3D), Iow temperature physical model of an industrial Hall-Heroult cell was developed, constructed and operated to obtain further understanding of fluid dynamic phenomena in operating reduction cells. A fluid-dynamic probe (pressure probe), data acquisition system and signal analysis techniques were developed to characterise various fluid-dynamic properties in both model and operational cells. The short wavelength metal pad interfacial deformation was measured in industrial trials and correlated with the anode voltage signal fluctuation. The results obtained illustrate that while the bubble release dynamics play an important role in the voltage signal fluctuation, the short period interfacial wave is also important. Operational parameters such as bath height and channel width were varied in the physical model and the effects on turbulence, alumina dispersion and bubble entrainment were determined. The results of these visualisation investigations suggest a feed point location where optimal alumina dissolution is achieved and suitable bath height/centre channel width geometries for the dissolution and dispersion processes. A numerical model of the model cell was also developed to generate results including fluid velocity vectors which were then compared with results obtained through flow visualisation experiments in both the 2D and 3D physical models. The weaknesses in the numerical model calculations of bath phase flow are highlighted. The pressure probe was also evaluated in a physical model of an aluminium treatment unit used in the removal of inclusions and dissolved species. It was found that the transition from well-dispersed to poorly dispersed gas bubble regimes could be characterised using this probe. The optimal operating parameters such as rotor speed and gas flow rate could be determined with the help of the pressure probe.